The transconductance of the transistors in integrated circuits decreases with increasing temperature of the substrate chip, largely as a result of decreasing carrier mobility. For circuits which must have good temperature stability, such as those used for measurements or certain timing functions, it is necessary to compensate for the change in the transconductance by correspondingly varying the bias current for the transistors. For this reason, there is a need for an integrated circuit transistor biasing sub-circuit with a reference current which varies as a function of the absolute temperature of the substrate. Such reference current can be generated in a current loop referred to as a PTAT (Proportional To Absolute Temperature) current source.
In a known type of current source, such as is described for example in U.S. Pat. No. 4,029,974 issued June 14, 1977 to Brokow and entitled "Apparatus for Generating a Current Varying with Temperature", a first, unity gain current mirror for supplying a current relatively independent of supply voltage fluctuations is connected head-to-tail, or input-to-output to a second, temperature-sensitive current mirror to form a regenerative current loop.
The first mirror has first and second PNP current transistors of matched junction areas with their emitters connected to a positive supply voltage through equal emitter resistors and with their bases connected together and tied to the collector of the second transistor. The second transistor is considered the input transistor, while the first transistor is considered the output transistor.
The second mirror has third, input and fourth, output NPN current transistors, with the fourth current transistor having a larger junction area than the third current transistor. The collectors of the first and third current transistors are connected together, as are the collectors of the second and fourth current transistors. The bases of the third and fourth current transistors are connected to each other and to the collector of the third current transistor. The emitter of the third current transistor is connected directly to a negative supply voltage, while the emitter of the fourth current transistor is connected to the negative supply voltage through a current-setting resistor which establishes the operating current for the loop.
With such an arrangement, the base-emitter current densities in the third and fourth current transistors of the second mirror will be unequal. The resulting difference between their base-emitter voltages will be proportional to the absolute temperature and will appear across the current-setting resistor.
It is a major shortcoming of the above-described type of current source that while the two equal emitter resistors for the first mirror can have their resistance vary with changes in temperature without thereby causing a significant change in the loop current, the resistance of the current-setting resistor connected between the emitter of the second mirror output transistor and the negative supply voltage must have a zero temperature dependence if the proportionality of the current to the absolute temperature is to be preserved.
Resistors which can be integrated into the circuit on the chip, however, typically have a temperature coefficient on the order of about +2000 ppm/.degree.C. (parts per million per degrees Celsius). In comparison, at room temperature the ideal PTAT characteristic itself is on the order of +3300 ppm/.degree.C. A current source as described above using such diffused resistors would thus have a net temperature coefficient of approximately 3300-2000=1300 ppm/.degree.C., representing a deviation of over 60 percent from the desired PTAT characteristic. For this reason, in order to have reasonable adherence to the PTAT characteristic, it has been necessary to use an off-chip discrete resistor instead of an integrated one.
The provision of a current-setting resistor off-chip is undesirable, primarily because it requires the use of at least one terminal of the packaging for the chip. Such passive use of a terminal limits the extent to which the integrated circuit can be addressed actively for its available functions.